1. Field of the Invention
This invention relates to a process and an apparatus suitable for forming a deposited film, above all a functional film, especially an amorphous or crystalline deposited film to be used for semiconductor devices, photoelectric energy transducers (solar battery, photosensitive drum, image sensor, etc.), or photovoltaic devices, etc.
2. Related Background Art
Usually, for formation of a silicon (Si) film at low temperature, various CVD methods such as the plasma CVD method, the optical CVD method, etc., the vacuum vapor deposition method, the reactive sputtering method, the ion plating method, etc. have been attempted, and, in general, the plasma CVD method has been widely used and industrialized.
Most of the deposited films obtained by the plasma CVD method are amorphous silicon (A-Si) films, and there is room left for further improvement of overall characteristics with respect to electrical, optical characteristics and fatique characteristics or uniformness, bulk productivity, etc. Also, according to the plasma CVD method, it is difficult to deposit a polysilicon film of good quality on a glass substrate.
The reaction process in formation of silicon deposited film according to the plasma CVD method generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and not a few ambiguities exist in its reaction mechanism. Also, there are involved a large number of parameters for formation of its deposited film (e.g. substrate temperature, flow rates and their ratios of gases introduced, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, gas discharging speed, plasma generation system, etc.), and the plasma formed by combination of such a large number of parameters may sometimes become unstable to frequently result in markedly bad influences on the deposited film formed. Besides, the parameters inherent in the devices must be chosen for each device, and it has been difficult under the present situation to generalize the production conditions.
As a substitute for the plasma CVD method in which many ambiguities are involved in the reaction mechanism, there has been proposed HR-CVD in which a plural number of gases are introduced into a film forming space, and also some of the gases are previously activated by an energy of light, heat, plasma, etc., thereby depositing a film on a substrate arranged in the film forming space (Japanese Laid-open Patent Publication No. 41047/1985).
According to such an invention, for example, when forming an amorphous silicon (A-Si) film, there is adopted a method in which SiF.sub.2 gas which is a precursor and H* (hydrogen radical) which is an active species are permitted to be associated with each other to form A-Si on the substrate.
The particularly advantageous point of this method is that a variety of Si films ranging from a Si film containing microcrystalline phase which is one form of amorphous silicon to a polycrystalline Si(poleg-Si) can be prepared depending on the concentration of H*.
In the conventional HR-CVD method, however, we often have been confronted with difficulties, in case of introducing into a deposition space a plurality of precursors depending on the number of elements constituting a film to be formed, in attaining a desired ratio of the elements incorporated into the film.
For example, as described above, it has been possible, as for independently, to associate SiF.sub.2 gas as a precursor with H* as an active species to form an A-Si film on a substrate, or similarly to associate GeF.sub.n (n=1,2 or 3) gas as a precursor with H* as an active species to form an A-Ge film, but formation of a diposited film containing Si atoms and Ge atoms at a ratio of 1:1 is not easily performed and a film rich in Si is formed inevitably. In order to improve this, some attempts of varying the introducing ratio of SiF.sub.2, GeF.sub.2 and H* gases have been practiced, but in most cases, a deposited film of desired composition could not be obtained.
Thus, although HR-CVD has various advantages over the conventional plasma CVD, there is still room for improvement particularly in case of forming deposition films constituted of plural kinds of elements.